1. Field of the Invention
The present invention relates to a method of stud welding and an improved stud welding chuck.
2. Description of the Prior Art
Stud welding is a process which finds widespread industrial application for fastening metal studs on metal surfaces. A particular advantageous feature of stud welding in attaching studs and other fasteners to metal surfaces is that all operations can be performed from one side of the metal surface involved. Therefore, studs can be fastened to metal surfaces which are not accessible from the opposite side.
A stud welding machine typically employs an alternating current to direct current transformer as a source of electrical power. The direct current electrical power is typically provided through electrical cables to a hand-held stud welding gun, shaped somewhat like a pistol. The stud welding gun carries a chuck into which elongated stud fasteners are inserted for placement on a metal surface. The stud chuck is constructed of metal, as are the stud fasteners and the surfaces to which the stud fasteners are to be attached.
Each stud fastener is equipped with an elongated, projecting shank and a base to be fastened to a metal surface. The free end of the shank is inserted into the stud welding chuck, and a short, but very large surge of current is directed through the chuck and through the shank of the workpiece to the workpiece base and to the metal surface to which the stud is to be welded. The surge of electrical current effectuates localized melting of the base of the stud and of the metal surface against which the stud is positioned and to which it is to be attached. The molten metal rapidly cools leaving the stud shank projecting from the surface to which it is then attached. The stud welding gun is then drawn back from the metal shank, thereby freeing the stud welding chuck from the welded stud shank. Another stud is inserted into the stud welding gun and the process is repeated for any number of studs to be welded.
There are two major types of stud welding processes. In capacitor discharge stud welding a very large capacitor is employed to store direct current electrical power. One end of the stud to be welded is inserted into a snugly fitting stud welding chuck. The stud welding chuck is thereby used to hold the stud in position during the welding operation. A small, axial pin projects outwardly from the base of the stud, away from the stud welding chuck. The stud is pressed against the metal surface at the location at which it is to be attached. A spring mechanism acting between the chuck and the stud welding gun body urges the stud toward the surface to which it is to be welded. Depression of the stud welding gun trigger discharges the capacitor through the chuck and through the stud to the metal surface. The brief but large surge of electrical current melts the pin on the base of the stud and the area of the metal surface in contact therewith. As melting is initiated and progresses the spring acting between the stud welding gun body and the chuck urges the stud toward the metal surface. The molten metal spreads across the interface between the metal surface and the base of the chuck. Following the capacitor discharge the molten metal cools, thereby leaving the stud firmly welded to the metal surface. The stud welding gun is drawn back from the upstanding stud shank, since the stud shank is held in the chuck only by frictional engagement.
Another form of stud welding is arc welding. Arc welding is used in heavier duty applications for larger fasteners and rougher work surfaces, as contrasted with capacitor discharge stud welding. In an arc welding system power is derived from a rectifier or a motor generator. As in the capacitor discharge stud welding process, the shank of the stud to be arc welded is inserted into the stud welding chuck in the stud welding gun. The base of the fastener to be welded is pressed against the work surface with spring loading. A ceramic shield on the stud welding gun surrounds the base of the stud to be welded. When the stud welding gun trigger is pulled the stud welding chuck is momentarily lifted slightly from the surface to which it is to be welded, as a surge of direct current electric power is released to arc between the base of the stud and the surface to which the stud is to be fastened. The power dissipated in the arc melts the base of the stud and the adjacent area of the work surface. The ceramic shield aids in concentrating the heat at the interface between the base of the stud and the metal surface to which the stud is to be attached to ensure a firm, secure weld. As the base of the stud and the area of the metal surface adjacent thereto are melted, the chuck is again spring biased toward the metal surface, thereby plunging the stud into the molten metal. The molten metal rapidly cools, thereby leaving the stud projecting from the metal surface.
A number of different models of stud welding systems are manufactured by Nelson Stud Welding which is located at 28th St. and Toledo Ave., Lorain, Ohio 44055.
One problem which has persisted in employing stud welding as a means for attaching metal studs to metal surfaces has been that a different stud welding chuck has been necessary for each different diameter stud to be welded. A stud welding machine operator must therefore change the stud welding chuck in the stud welding machine each time studs of a different diameter are to be welded. Conventional chucks are constructed with a tapered, frusto-conical end, which is inserted into a correspondingly tapered recess in a spindle in the stud welding gun. To change a chuck, the user must insert a screwdriver in a slot in the stud welding gun behind the chuck and use the screwdriver as a lever to push the chuck out of fricitional engagement with the spindle. A new chuck, designed for the diameter of stud fastener to be stud welded, is then placed into the tapered recess in the spindle and tapped lightly to ensure that the chuck has properly seated. The stud to be welded is then placed into the chuck. A spacing leg must then be adjusted for the new chuck. The spacing leg carries a footpiece which limits the extent to which the stud welding gun will be forced toward the surface upon which studs are to be attached during the stud welding process. The leg carrying the footpiece must be adjusted so that the base of the stud, and any projection therefrom, extends approximately 1/8 inch beyond the bottom of the footpiece.
The foregoing steps in changing chucks are quite time consuming. It is evident that when studs of different diameters are to be attached to a metal surface there is a considerable loss in productivity due to the time involved in changing chucks.
A further disadvantage of conventional stud welding chucks is that they tend to wear out relatively rapidly. There is considerable friction on the inner surface of the chuck barrel when studs are inserted for placement, and when the stud welding gun is withdrawn from the installed stud. Once the barrel has worn to a certain point, the studs for which the chuck is designed are too loose. Arcing then occurs at the interface of the stud with the chuck barrel during the welding process. Once arcing commences, the chuck rapidly burns out. A typical prior art chuck will last for approximately 3,000 welds.